Many road-going vehicles, particularly larger industrial and/or commercial vehicles such as buses, trucks, recreational vehicles and the like, are manufactured using a chassis which includes an internal space frame or skeleton upon which the outer body structure is subsequently installed. This frame is typically composed of a plurality of tubular structural members which are interconnected by welding, riveting, bolting, etc, such as to produce the frame structure of the vehicle. These tubular members are most often hollow tubes having a circular, square or rectangular cross-sectional shape.
Public transportation vehicles such as buses are expected to survive for extremely long life cycle periods, during which time the vehicle, and therefore its internal frame, is exposed to high stresses and very high mileage. As such, the welded joints formed between the interconnected tubular members of the vehicle's frame are particularly exposed high stresses and repeated fatigue forces, and it is at these joints where concentration of stresses occur. Therefore, it is the joints themselves between the tubular members which are prone to potential weakening, leading eventually to failure of the joints in areas of the frame which are exposed to the greatest concentration of stresses. Adjacent tubes which meet at right angles and are butt welded together are often particularly prone to such high stress concentration at the welded joint therebetween.
However, such right angle joints between tubes are unavoidable in the frame structure of a bus, given that relatively large openings between tubes of the frame must remain free of reinforcement structure, which openings are adapted for receiving windows or doors of the vehicle therein. As such the welded joints between those tubes which are fastened together around such large openings, i.e. the joints which fasten together the tubes which form the window or door frame, are particularly exposed to stress concentrations and are therefore particularly prone to weakening, damage and/or failure given sufficient forces for a sufficiently long period of time.
Attempts to reinforce certain regions of such tubular vehicle frames have been made, for example by providing reinforcement gussets between adjacent welded tubes. However, the added strength provided by this simple gusset reinforcement has not proved to be sufficient. Alternately, other means of improving the strength of such structures and therefore minimizing the effects of stresses thereof have been attempted, such as by using larger diameter tubes or by simply attempting to avoid butt welded joints between tubes which are oriented perpendicularly to each other. However, penalties exists which all of these attempted methods, including weight and cost penalties, while nevertheless failing to provide a significant increase in strength. Generally, designers of such commercial road vehicles are reluctant to move away from using hollow tubular frame members having a rectangular or square cross-sectional profile, as such tubes typically provide good strength to weight ratios, while being relatively inexpensive to manufacture and relatively easy to assemble and weld together.
It would therefore be desirable to be able to improve the overall strength of a tubular frame structure for a bus or other large road vehicle, and more particularly to be able to improve the strength of joints between tubes in certain regions of such tubular frames, by reducing the stress to which such regions and/or joints are exposed during operation of the vehicle.